As far as the magnitude of transpiration is concerned, Meyer (1956) had reported that some of the herbaceous plants, under favourable conditions, transpire the entire volume of water which a plant has and it is replaced within a single day. A tropical palm under well watered conditions may lose as much as 500 litres of water per day. Daily loss of water by an apple tree may be 10-20 litres. A maize plant may lose 3-4 litres of water per day.
Crotolaria juncea evaporates 27 kg of water in its life cycle of 140 days and Helianthus annuus about 56 kg of water. According to estimates at least 1000 gallons of water are lost every month during summer by a 9-10.5 metres high tree. A birch tree with 200,000 leaves evaporates 300-400 kg of water on a hot day, whereas a 15 years old beech tree evaporates 75 kg water per day during summer. A beech forest of 400-600 trees evaporates some 20,000 barrels of water per day.

Transpiration occurs from all aerial parts of a plant and water is stores in large amount of leaves. However, most of the transpiration takes place through the leaves. It is called foliar transpiration. Stems transpire very little. Transpiration from stem is called cauline transpiration. Depending upon the plant surface involved, transpiration is of three types – cuticular, lenticular and stomatal transpiration.

# (i) Cuticular transpiration : Cuticle is a layer of wax like covering on the epidermis of leaves and herbaceous stems. It provides a relatively impermeable covering. If it is thin, upto 20 percent of the total transpiration may take place through it, but with the increase in its thickness (e.g., in xerophytes), the water vapour loss is reduced.

# (ii) Lenticular transpiration : Lenticles are the areas in the bark of woody plants which are filled with loosely arranged cells known as complementry cells. Loss of water vapour through lenticels is called lenticular transpiration. It amounts to about 0.1 percent of the total water loss through transpiration.

# (iii) Stomatal transpiration : Stomata are minute pores in the epidermis of leaves, young green stems. The loss of water vapour, which occurs through stomata is called stomatal transpiration. It amounts 80-90 percent of the total water vapour loss from the plants. It is the most common type of transpiration. Arora and Lamba (1982) have reported the presence of stomata on fruit wall of Raphanus sativus var. caudatus and Brassica oleracea var. botrytis.

Stomata are the microscopic openings most commonly found in the leaves. These may be present in young stems and sometimes even in fruits (e.g., citrus, banana, cucumber, etc.). Each stomatal opening is surrounded by two specialised epidermal cells, called as the guard cells.
Because of their small size, the guard cells are rapidly influenced by turgor change and thus regulate the opening and closing of stomata. The guard cells of dicot leaves are kidney-shaped or raniform whereas those of monocots (family Gramineae) are dumbel-shaped or elliptical. The guard cells are surrounded by epidermal cells called as the accessory cells or subsidiary cells. These are different from the normal cells of epidermis having chloroplasts. The stoma with subsidiary cells is called stomatal apparatus. Each stoma leads into a air space called sub stomatal cavity. Both kidney shaped and dumbel-shaped guard cells have been reported in Cyperus. Each guard cell has a thin layer of cytoplasm along the cell wall and a large vacuole. Its cytoplasm contains a distinct nucleus and several chloroplasts. The cell wall of guard cells around the stomatal pores are thickened and inelastic due to presence of a secondary layer of cellulose. Here the cellulose microfibrils are radially arranged and they radiate away from the pore. Rest of the wall is thin, elastic and semipermeable.

The stomata differ in their distribution on the two surfaces of the leaf. The leaves are classified into following types on the basis of stomatal distribution on them :

# (i) Epistomatic (Water Lily type) : Stomata are present only on the upper epidermis of leaves. These are found in water Lily, Nymphaea and many other floating hydrophytes.

# (ii) Hypostomatic (Apple or Mulberry type) : Stomata are present only on the lower surface of leaves. e.g., Apple, mulberry, peach and walnut.

# (iii) Amphistomatic : Stomata are present on both the surfaces of leaves. It can further be subdivided into two types :
- (a) Anisostomatic (Potato type) : The number of stomata is more on the lower surface and less on the upper surface. In other words, the lower surface is multistomatic and the upper surface is paucistomatic. Such leaves are also called as dorsiventral leaves. e.g., Potato, tomato, bean, pea, and cabbage.
- (b) Isostomatic (Oat type) : The stomata are equally distributed on both the surfaces of leaves. These leaves are also called as isobilateral leaves. These are found in monocots e.g., Oat, maize, grasses, etc.

# (iv) Astomatic (Potamogeton type) : Stomata are either absent altogether or vestigial. e.g., Potamogeton and submerged hydrophytes.

Loftfield (1921) classified the stomata into four types, depending upon the periods of opening and closing.
(i) Alfalfa type (Lucerne type) : The stomata remain open throughout the day but close during night, e.g., Pea, bean, mustard, cucumber, sunflower, radish, turnip, apple, grape.
(ii) Potato type : The stomata close only for a few hours in the evening, otherwise they remain open throughout the day and night e.g., Cucurbita, Allium, Cabbage, Tulip, Banana etc.
(iii) Barley type : These stomata open only for a few hours in the day time, otherwise they remain closed throughout the day and night, e.g., Cereals.
(iv) Equisetum type : The stomata remain always open through out the day and night. e.g., Amphibious plants or emergent hydrophytes.

Opening and closing of stomata occurs due to turgor changes in guard cells. Due to endosmosis, an increase in turgor of guard cells takes place which finally results in stretching and bulging out of their outer thin walls. This results in the pulling apart of the opposed inner thicker walls creating an opening or pore in guard cells of stomata. When the turgor pressure of guard cells decreases, inner walls sag, leading to closure of space between them. This is due to the loss of water (exosmosis) from guard cells, resulting in thicker walls to move closer and finally shut the opening. The transpiration is regulated by the movement of guard cells of stomata.

Several theories have been put forth to explain the opening and closure of stomata. Which have been discussed below :
# (i) Photosynthetic theory : According to Von Mohl (1856) the chloroplasts present in guard cells prepare osmotically active substances by photosynthesis. As a result, their osmotic pressure increases and their turgor pressure increases due to endosmosis. This results in opening of stomata.
This theory is not accepted because in many cases, chloroplasts of guard cells are poorly developed and incapable of performing photosynthesis.

# (ii) Starch ⇄ sugar interconversion theory : According to Lloyd (1908), turgidity of guard cells depends upon interconversion of starch and sugar. This fact was supported by Loftfield (1921) who found that guard cells contain sugar during day time when they are open and starch during night when they are closed. Later Sayre (1926) observed that stomata open in neutral or alkaline pH which prevails during day time due to constant removal of CO2 by photosynthesis. They remain closed during night when there is no photosynthesis and due to accumulation of CO2, carbonic acid is formed which causes the pH to be acidic, Sayre thus proposed that interconversion of starch and sugar is regulated by the pH. Sayre's hypothesis was supported by Scarth (1932) and Small et al (1942). This hypothesis was further supported by detection of the enzyme phosphorylase in guard cells by Yin and Tung (1948). This enzyme is responsible for starch-glucose interconversion.

(i) During day time due to rapid rate of photosynthesis, the concentration of CO2 decreases in the guard cells. As a result their pH is increased. At higher pH, starch in the guard cells is converted into organic acid by the enzyme phosphoenol pyruvate carboxylase (PEPC). This enzyme was discovered by Willmer etal. (1973). It can convert several others carbohydrate into organic acids.
(ii) The organic acid (e.g. malic acid) dissociates into H+-ions (protons) and malate ions.
(iii) The protons (H+) are actively transported into subsidiary cells in exchange for K+ with the help of an energy (ATP) driven H+-K+-pump. The uptake of K+-ions is balanced by uptake of Cl and the negative charge on malate-ions.
(iv) Increased concentration of K+ and malate ions in the guard cells increases the O.P. of guard cells.
(v) Water enters from adjoining subsidiary cells by endosmosis.
(vi) Turgor pressure of guard cells increases. Turgidity of guard cell is controlled by potassium, chloride and malate.
(vii) Stomata open.

According to Cowan et.al. (1982) closure of stomata depends upon abscisic acid (ABA) which is in fact an inhibitor of K+-uptake. It becomes functional in presence of CO2 or in acidic conditions (low pH).
(i) During night photosynthesis stops which results in increased concentration of CO2 which causes lowering of pH.
(ii) At lower pH, ABA inhibits K+-uptake by changing the permeability of guard cells.
(iii) The K+-ions now start moving out of the guard cells which results in lowering of the pH.
(iv) At low pH, organic acids are converted back into starch by PEPC.
(v) The O.P. of guard cells decreases and water moves out of them into subsidiary cells by the process of exosmosis, thus decreasing their turgor pressure.
(vi) The guard cells become flaccid and the stomata closed.

(i) External factors
# (a) Atmospheric humidity : If the atmosphere is humid, it reduces the rate of transpiration. When the air is dry, the rate of transpiration increases.

# (b) Temperature : It affects the rate of transpiration only indirectly. Increase in the temperature of the air decreases the humidity of the air and therefore more water is vapourised and lost from the transpiring surface. The lowering of the air-temperature, on the other hand, increases the humidity and rate of water-loss as well.

# (c) Light : Light affects the rate of transpiration due to its effect on temperature and photosynthesis. During daytime stomata open wide but during night they close. Moreover, during the daytime the light also helps in raising the temperature. Thus increased temperature and presence of wide open stomata increase the rate of transpiration. Light is the most important factor in the regulation of transpiration.

# (d) Atmospheric pressure : The rate of transpiration is inversely proportional to the atmospheric pressure.

# (e) Available soil water : If the available water in the soil is not sufficient the rate of transpiration is decreased. Under internal water deficiency the stomata are partially or completely closed.

# (f) Wind velocity : A transpiring surface of leaf continuously adds water vapours to the atmospheric air. Once the immediate area becomes saturated, it reduces the rate of transpiration. Wind velocity removes the air of that area, which is replaced by fresh air and result in an increases in the rate of transpiration. Wind velocity is measured by anemometer.

(ii) Internal factors/Plant factors

# (a) Leaf area : If leaf area is more, transpiration is faster. However, the rate of transpiration per unit area is more in smaller leaves than in larger leaves due to high number of stomata in a small leaf. Number of stomata per unit area of leaf is called stomatal frequency.
here, I = Stomatal index
S = No. of stomata per unit area
E = No. of epidermal cells in unit area.

# (b) Leaf structure : The anatomical features of leaves like sunken or vestigial stomata; presence of hair, cuticle or waxy layer on the epidermis; presence of hydrophilic substances such as gums, mucilage etc. in the cells; compactly arranged mesophyll cells etc. help in reducing the rate of transpiration.

# (c) Root shoot ratio : According to Parker (1949) the rate of transpiration is directly proportional to the root-shoot ratio.

# (d) Age of plants : Germinating seeds show a slow rate of transpiration. It becomes maximum at maturity. However, it decreases at senescence stage.

# (e) Orientation of leaves : If the leaves are arranged transversely on the shoot they lose more water because they are exposed to direct sunlight. If placed perpendicularly they transpire at slower rate.

Significance of transpiration : The advantages and disadvantages of transpiration are discussed below :

# (i) Advantages
(a) Transpiration is important for plants because it directly influences the absorption of water from the soil.
(b) Transpiration exerts a tension or pull on water column in xylem which is responsible for the ascent of sap.
(c) Transpiration helps in the movement of water and minerals absorbed by the roots to the other parts of the plant.
(d) The evaporation of water during transpiration contributes to the cooling of leaves (and also the surrounding air) and protects leaves from heat injury particularly under conditions of high temperature and intense sunlight.

# (ii) Disadvantages
(a) Transpiration often results in water deficit which causes injury to the plants by desiccation.
(b) Rapid transpiration causes mid-day leaf water deficit (temporary wilting). If such condition continues for some time, permanent water deficit (permanent wilting) may develop, which causes injury to plants.
(c) Many xerophytes have to develop structural modifications to reduce transpiration. These modifications are extra burden on the plants.
(d) Excessive rate of transpiration leads to stunted growth of plants.
(e) Deciduous trees have to shed their leaves during autumn to check transpiration.
(f) Since approximately 90 percent of absorbed water is lost through transpiration, the energy used in absorption and conduction of water goes waste.
Besides all the above mentioned disadvantages, the process of transpiration is unavoidable, because of the anatomical structure of the leaves. Since stomata are required for gaseous exchange in photosynthesis and respiration, the loss of water through them cannot be avoided. Therefore, Curtis (1926) truely called 'transpiration as a necessary evil'.

Anti-transpirants : Most of the water absorbed by plants is lost to the atmosphere by transpiration and hence water use by plants is very inefficient. In recent years efforts have been made to improve the efficiency of water use by the plants. One of the approaches is to reduce transpiration by the application of certain chemical substances. 'The chemical substances which reduce transpiration (by increasing leaf resistance to water vapour diffusion) without affecting gaseous exchange, are called anti-transpirants'. Anti-transpirants are of two types metabolic inhibitors and film forming anti-transpirants.

#(i) Metabolic inhibitors : They reduce transpiration by causing partial closure of stomata, without influencing other metabolic processes, the most important of these inhibitors are phenyl mercuric acetate (PMA) and abscissic acid (ABA).

# (ii) Film forming anti-transpirants : They check transpiration by forming a thin transparent film on the transpiring surface. They are sufficiently permeable to carbon dioxide and oxygen to allow photosynthesis and respiration, but prevent movement of water vapour through them. The important chemicals of this group are silicon emulsion, colourless plastic resins and low viscosity waxes.